Engineering Chemistry S Datta

NUCLEAR CHEMISTRY 47
Artificial Radioactivity
If an atom of a non-radioactive element is bombarded by a powerful particle, it is converted
to a radioactive atom, e.g. if we bombard 27 13 Al with an alpha (α) particle, we get radioactive
phosphorus which is ultimately converted to silicon.
27
13
4
2
30
15
1
0
Al + He ⎯→ P + n
30 30
P ⎯→ Si + e.
15
This phenomenon is known as Artificial Radioactivity.
Atomic Mass Unit
Masses of atoms are usually expressed in atomic mass units (a.m.u.).
19
1 a.m.u. = 1
12
So, 1 a.m.u. = 1
12
where N (Avogadro number) = 6.023 × 10 23
Mass–Energy Equivalence
1 a.m.u. =
0
1
12
× 6C
12 1
× =
N g N g
1
6.
023 × 10
23
−24
= 166 . × 10 g.
In 1905, Einstein established from theoretical standpoint that mass and energy are
mutually convertible. The famous equation in this regard is
E = mc 2
where E = energy ; c = velocity of light.
The year 2005 has been declared as year of physics to observe the centenary of this
famous equation.
Highlight:
E = mc 2
= 1 × (3 × 10 10 ) 2 g cm 2 sec –2
= 9 × 10 20 g cm 2 sec –2
= 9 × 10 20 ergs
= 9 × 10 13 joules.
So, 1 g of matter is equivalent to 9 × 10 13 J of energy.
Mass-Energy conversion factor
We know that, 1 eV = 1.6 × 10 –12 ergs.
So, 1 MeV = 1.6 × 10 –6 ergs
and 1 a.m.u. = 1.66 × 10 –24 g.
∴ 1 a.m.u. = 1 . 6 × 9 × 10
−6
16 . × 10
20
≈ 931 MeV.
Nuclear Fission
In 1939, German scientists Hahn and Strasman bombarded uranium atom with neutrons and
obtained two elements with atomic numbers 56 and 36.